This invention concerns the dehydroisomerization of hydrocarbons. These hydrocarbons may be aliphatic in nature, but the process of this invention is especially suitable for the dehydroisomerization of alkylcyclopentanes, and of hydrocarbon feedstocks containing alkylcyclopentanes, into aromatic hydrocarbons. In a preferred embodiment, it concerns the dehydroisomerization of methylcyclopentanes into benzene and toluene in the presence of a specific catalyst system.
Alkylcyclopentanes are found as components of light straight run gasoline, and are contained in other gasoline fractions, such as naphtha fractions resulting from thermal and catalytic conversion of petroleum. Typically, saturated gasoline or naphtha fractions are treated or upgraded to improve their anti-knock characteristics. One means of upgrading such streams is by the well known process of reforming, wherein naphthenic hydrocarbons such as cyclohexane compounds are dehydrogenated to aromatics. Subjecting five membered naphthene ring compounds such as alkylcyclopentanes to conventional dehydrogenation or reforming catalysts and processing conditions, however, causes the formation of minor amounts of cyclic mono-olefins and di-olefins in addition to the production of undesirable amounts of carbonaceous deposits on the catalyst in view of the susceptibility of naphthenes to cracking. While some isomerization of alkylcyclopentanes to cyclohexanes may occur, the overall effect has been to downgrade such cmponents to less valuable products having little or no value as gasoline blending components. Likewise, the practice of separating five membered naphthene ring hydrocarbons for separate isomerization of such compounds to cyclohexanes followed by dehydrogenation of the cyclohexanes to aromatics, is at best a poorly selective process involving the use of a plurality of catalysts and reaction zones, which is economicaly unattractive from a processing standpoint.
It is known that hydrogen mordenite is a suitable substrate for catalytically active metals, and has been used in a variety of hydrocarbon isomerization and conversion processes. See, for instance, U.S. Pat. Nos. 3,432,568; 3,475,345; 3,507,931; 3,574,092; 3,409,685 and 3,709,817; and a paper by Minachev et al. in Izv. Akad. Nauk SSSR, Ser. Khim. 8, 1737-42 (August 1969). It is, however, a problem that hydrogen mordenite itself exhibits considerable cracking activity, leading to undesirably low yields of isomerized product, and wastage of feedstocks.
Attempts have been made in the past to suppress the cracking activity of a catalyst by the use of a catalyst poison. For example, British Pat. No. 1.009,943 shows that the course of reactions carried out on zeolitic aluminosilicate catalysts can be controlled by the use of appropriate catalyst poisons. For instance, dehydration, hydration, cracking and olefin isomerization processes are all acid-catalyzed and suppressed by basic catalyst poisons; while hydrogention is catalyzed by transition metals and poisoned by sulphur, nitrogen and phosphorus compounds.
British Pat. No. 1,266,781 describes a process wherein the cracking activity of the outer surface of a porous zeolitic molecular sieve catalyst is decreased by poisoning with ammonia, an alkylamine or carbon disulphide at a temperature from 400 to 1000.degree. F., while the activity of the inner surface of the pores is not substantially decreased. As a result, liner hydrocarbons, such as n-hexane, can still be cracked, while the more bulky branched hydrocarbons, such as 3-methylpentane, cannot enter the pores and are not cracked.
An object of this invention is to provide a process for the hydroisomerization of hydrocarbons, and especially the dehydroisomerization of alkylcyclopentanes into aromatic products, in which cracking of the feedstock is minimized.
It has now surprisingly been found that, by working under carefully controlled temperature conditions, not only is the cracking activity of a hydrogen mordenite catalyst suppressed by contacting the catalyst with ammonia, but the isomerization reaction, which is also known to be acid-catalyzed, and hence poisoned by bases, is maintained. It is thereby possible to provide a hydrocarbon dehydroisomerization process of enhanced selectivity.